This relates generally to reflective polarizers which transmit one polarization and reflect the other polarization. More particularly, this application relates to reflective polarizers having an extended red band edge in the off-axis transmission spectrum to reduce off axis color hue, or to select a particular hue.
One important use for reflective polarizers is in a light-recycling mode to provide brightness increase (gain) in liquid crystal display (LCD) applications. In these applications, the reflective polarizer is used in conjunction with a light-recycling cavity, a light source, and a liquid crystal panel. Maximum luminance is achieved when the reflective polarizer is used in a brightness enhancement mode, such that light of the reflected polarization is "recycled" into the transmitted polarization by the reflective polarizer in combination with the light recycling cavity. Examples of such light recycling systems are described in copending and commonly assigned U.S. patent application Ser. No. 08/402,134, which is incorporated herein by reference.
The off axis color (OAC) problem with multilayer-type reflective polarizers is described in copending and commonly assigned U.S. patent application Ser. No. 08/402,041, which is incorporated herein by reference. OAC is also a problem with other types of reflective polarizers, such as cholesteric reflective polarizers. The multilayer reflecting polarizers described in the above mentioned U.S. patent application Ser. No. 08/402,041 show a very red or yellow OAC. In other words, the OAC has a distinct reddish or yellowish "hue". This is because the red band edge of the off axis transmission spectrum is shifted toward the blue. As a result, almost all p-polarized red light (600 nm and greater) is transmitted at off normal incidence, but some of the blue and green is reflected, giving the light at off normal incidence a definite reddish appearance. This hue is very objectionable in some applications, such as in the backlit LCD systems, where color control is highly important.